This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 and/or 365 to (attorney docket No. Serie 5540) filed in France on Dec. 20, 2000; the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a process of oxidation, of the wet oxidation or ozonization type.
The present invention particularly relates to the technology of wet oxidation, which is conventionally implemented in various types of gas/liquid reactors, and particularly bubble columns and tubular reactors, but also agitated reactors, conventional or not.
2. Description of the Related Art
A process of the wet oxidation type is in particular described in an article submitted to the JIE (International Days of Water) of Poitiers in 1998 and entitled: xe2x80x9cATHOS, an innovative solution for the elimination of sludgesxe2x80x9d (Proceedings JIE Poitiers 23-35 September 1998, Vol. 2, p. 69). Wet oxidation processes generally operate in the following manner: the treatment is carried out with air as oxidant, at 250-270xc2x0 C. and under 85-120 bars, in reactors having a height/diameter ratio comprised between 5 and 20. There are usually used for these plants, reactors of the bubble column type in which the oxidant gas is injected at the base, co-current or counter-current to the liquid. The height of the column is a function of the yields of gas/liquid transfer. The transfer coefficients are low (generally 10xe2x88x924 to 10xe2x88x922 sxe2x88x921), making these reactors particularly suitable for reactions having slow kinetics (chemical conditions). The reaction volumes are considerable and the residence times are long (generally several hours).
The above-cited article likewise describes a process known under the name of xe2x80x9cAqueous Phase Oxidation Processxe2x80x9d, which consists of treating sludges from biological plants with pure oxygen in a tubular reactor placed in a deep borehole of 1,200 m, constituted by two concentric principal tubes, one for injecting the thick sludge and oxygen, and the other for the ascent of the treated sludge.
A conventional stirred reactor (Stirred Tank Reactor or STR) is described in the article by M. Lawrence M. Litz, in CEP, November 1985, pp. 36-39, entitled: xe2x80x9cA Novel Gas-Liquid Stirred Tank Reactorxe2x80x9d. Within such a stirred reactor, the distribution of gases is generally effected by a perforated torus placed at the base of the reactor, below a mechanical agitator designed, in principle, to disperse the gas in the whole of the reaction medium. This type of reactor enables higher transfer coefficients to be obtained, as a function of, among other things, the speed of agitation and the nature of the moving body, leading to lower residence times than those for the above-cited two types of reactors.
A process is described in WO-A-99/04088 of oxidation of white or black liquors, effected under pressure and at high temperature in a stirred reactor equipped with a multi-blade stirrer permitting axial and radial gas/liquid mixing. Downstream of the said reactor there is disposed a gas/liquid separator intended to separate the undissolved gases from the oxidized liquor.
A process is described in WO-A-96/13463 of oxidation of effluents in a non-conventional reactor in the presence of a heterogeneous catalyst which acts in the gas phase formed above the liquid phase. The liquid phase is agitated by recirculation, by means of an external pump. An in-line mixer disposed in the recirculation loop intimately mixes the gas with the liquid phase.
Reactors are described in U.S. Pat. Nos. 4,328,175 and 4,454,077 which are equipped with downflow gas/liquid mixing means constituted by a helicoidal impeller which creates a vortex in the surface of the liquid phase. The gas is injected into the gas headspace of the reactorxe2x80x94the gas headspace being the space formed above the surface of the liquidxe2x80x94and is entrained and mixed with the liquid by a vortex effect within the liquid phase. The pumping output produced by the screw enables the gas/liquid mixture to be dispersed into the whole of the reactor volume.
There is described in U.S. Pat. No. 4,919,849 and EP-A-579 251 an agitated reactor equipped with a helicoidal impeller, in which reactor, above the agitating means of the impellerxe2x80x94axial downflow agitatorxe2x80x94oxidant gas injection tubes are provided, which are arranged horizontally below the minimum liquid level and, around a hollow portion of the agitation shaft. The gas, delivered at the end of the said tubes, is taken up by the descending pump output created by the helicoidal impeller (axial flow agitator) placed within the hollow cylinder. The impeller whose rotation ensures the formation of the desired vortex disperses the gas/liquid mixture toward the bottom of the reactor.
Described in EP-A-754 489 is a stirred reactor putting into effect a communication between the gas headspace and the body of the liquid phase. Injected into the gas headspace, the gas is aspirated into the liquid phase by means of L-shaped ducts. These are disposed around the stirrer shaft with their upper ends situated in the gas headspace, while their lower ends open behind, and in the periphery of, the blades of the said stirrer. The stirrer is a radial impeller with flat blades which at the same time ensures aspiration while generating a reduced pressure in the liquid phase and the mixing of the gas into the liquid.
The devices putting into effect a communication between the gas headspace and the body of the liquid phase ensure a recirculation of the gas in the reactor. In this type of apparatus, the recirculated gas flow can be correlated with a dimensionless number known as the xe2x80x9cmodified Froude numberxe2x80x9d (Fr*) defined by the following equation:       Fr    *    =                    N        2            xc3x97              d        2                    g      xc3x97      I      
with:
N: agitation speed in turns/second,
d: diameter of the moving agitator body in m,
g: acceleration due to gravity in m/s2,
I: depth of immersion of the impeller in m.
The apparatus described in EP-A-0 754 489 enables a value of the xe2x80x9cmodified Froude numberxe2x80x9d of 0.64 to be attained.
The process of the invention is a process of oxidation by an oxidizing gas of compounds dissolved or dispersed in a liquid medium. It concerns a process of oxidation which can equally be implemented in solutions and in dispersions. It is particularly suitable for the oxidation of sludges from biological purification plants, the oxidation of sulfur-containing minerals in hydrometallurgy, and the oxidation of sulfides in papermaking liquors. It also permits, for example, among other applications, improving the biodegradability of dissolved compounds, implemented in a reactor having a gas headspace. It is particularly applicable to wet oxidation or to ozonization.
The present invention comprises a process of oxidation, by means of an oxidizing gas, of a liquid contained in a reactor, which reactor has at least one means for introducing gas into the reactor and at least one means for introducing liquid into the reactor, at least one means for withdrawal of liquid, at least one means for agitating the liquid, at least one duct permitting the aspiration of gas present in the reactor and an opening into the liquid contained in the reactor. In the process, the liquid and the gas are introduced into the reactor in quantities such that a gas headspace is formed above the liquid contained in the reactor, in which, in the step of mixing the gas and liquid, the gas of the gas headspace is aspirated into the duct and introduced into the liquid present in the reactor at the level of the means for agitating the liquid, so as to dissolve at least a portion of the gas in the liquid, and in which, in a recovery step, the portion of the gas which is introduced into the liquid present in the reactor and which is not dissolved in the liquid is recovered in the gas headspace.